For the purpose of cleaning exhaust gases from automobiles a method has recently been adopted in which a known three way catalytic converter is employed for cleaning pollutants such as NO.sub.x, CO and HC components. In order to enable the three way catalytic converter to attain its highest cleaning function, it is necessary for the air-fuel ratio of an air-fuel mixture burning in an auto vehicle engine to be adjusted within a very narrow range (0.3-0.4), with the stoichiometric ratio being the central value in this range. However, the stoichiometric air-fuel ratio is somewhat richer than that desired to achieve a reduction in fuel consumption and unburned hydrocarbon emissions.
It has recently been noted that the content of the NO.sub.x emission is low at constant speed cruising and a deceleration engine condition because of the lower engine load. Consequently, an air-fuel ratio control arrangement for an internal combustion engine has been adopted in which, under the above conditions, the air-fuel ratio of an air-fuel mixture is maintained at a leaner level to attain a sufficient reduction in fuel consumption and to reduce the overall amount of emitted harmful components such as NO.sub.x, HC and CO. With this control arrangement, when a vehicle is operated under stop and go conditions requiring frequent acceleration for example, in heavy city traffic an additional fuel is supplied to control the air-fuel ratio of the air-fuel mixture to a value approximating the stoichiometric air-fuel ratio. Disposed in this air-fuel ratio control arrangement is a means for detecting the above driving conditions having a relation to the amount of NO.sub.x emitted from an engine. Therefore, a carburetor or fuel gas mixer of the engine is arranged so that under such low load or decelerating conditions an air-fuel mixture is made as lean as possible within the range wherein a misfire is not caused and is supplied to the engine which may be a gasoline engine or a liquefied petroleum gas engine (hereinafter referred to as "LPG engine"). Only when the above detecting means of the air-fuel ratio control arrangement detects conditions requiring the enrichment of the air-fuel ratio to the stoichiometric air-fuel ratio, does an air-fuel ratio sensor disposed in an exhaust pipe transmit a control signal to a feedback control circuit which causes an additional amount of fuel to be supplied to, for example, an intake manifold by an additional fuel supply nozzle so as to enrich the air-fuel mixture to the stoichiometric air-fuel ratio.
In the case of the LPG engine, in view of the fact that the propane-butane mixing ratio in the fuel is changed by the seasonal temperature change, it is necessary to set the air-fuel ratio under regular operation conditions to a leaner level of 18-19 although the stoichiometric air-fuel ratio is about 15.5. In the case where the difference between the stoichiometric air-fuel ratio and the air-fuel ratio of the predetermined lean mixture is relatively large and is about 3 and the above-mentioned control method is adopted, a large quantity of an additional fuel is supplied in an on-off manner due to occurence of a feedback control signal effecting the intended control of the air-fuel ratio causing surging in the output of the internal combustion engine and degraded engine performance. Moreover, after an additional fuel has been supplied by the additional fuel supply nozzle to compensate for the above-mentioned air-fuel ratio difference of about 3, the nozzle must perform a minute flow control corresponding to an air-fuel ratio of about 0.3 to about 0.4, so as to create exhaust conditions allowing the three way catalytic converter to act effectively. This means that the amount of fuel supplied from the nozzle is controlled in a flow amount range of 90 to 100% of the maximum flow amount. Therefore, the precision of the air-fuel ratio control by the additional nozzle is very poor. Because of these disadvantages the air-fuel control arrangement of the above-described type has not successfully been put into practical use.
Therefore, an object of the present invention is to provide an arrangement for controlling an air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine, whereby the foregoing disadvantages encountered with the conventional control arrangement are eliminated.
Another object of the present invention is to provide an arrangement for controlling an air-fuel ratio of an air-fuel mixture supplied to an automobile engine where, in under stable running conditions the air-fuel ratio in the air-fuel mixture is maintained at a level as lean as possible to attain a decreased fuel consumption, and under driving conditions requiring a richer air-fuel ratio a precise feedback control of the air-fuel ratio is possible in a very narrow air-fuel ratio range with the stoichiometric air-fuel ratio being the central value thereby allow harmful pollutants in the exhaust gas to be sufficiently cleaned.
Briefly, in the present invention, a lean air-fuel mixture is supplied to a gasoline engine or LPG engine through a carburetor or fuel gas mixer and, when the air-fuel ratio is enriched to the stoichiometric air-fuel ratio by the feedback control, a passage of an additional fuel supply system communicated with the carburetor or mixer is opened in response to the output of the above-mentioned engine operation condition detecting means to enrich the air-fuel ratio to a level slightly leaner than the stoichiometric air-fuel ratio. By controlling the quantity of the fuel to be supplied by the additional fuel supply nozzle according to the output of an air-fuel ratio sensor, the air-fuel ratio of the lean air-fuel mixture to be supplied to the engine in which the air-fuel ratio has been controlled to a level close to the stoichiometric air-fuel ratio is further minutely and precisely adjusted to the stoichiometric air-fuel ratio by feedback control.